Hitherto, for illuminating plants OSRAM has used mercury low-pressure discharge lamps in the form of fluorescent lamps which have a phosphor coating comprising the two phosphors listed above, namely BaMgAl10O17:Eu and Gd(Zn,Mg)B5O10:Ce,Mn, as well as a tin-doped strontium magnesium orthophosphate phosphor of the (Sr,Mg)3(PO4)2:Sn type in a ratio of approximately 22% by weight: 17% by weight: 61% by weight. However, the use of the (Sr,Mg)3(PO4)2:Sn phosphor has two drawbacks:    1. The grain size of the (Sr,Mg)3(PO4)2:Sn phosphor is significantly larger than that of the other two phosphors. This leads to a considerable color gradient along the lamp, in particular in the case of long lamps with a high wattage of greater than 36 W.    2. The tin doping of the (Sr,Mg)3(PO4)2:Sn leads to a high mercury consumption, which in the case of T8 fluorescent lamps with a tube diameter of 26 mm amounts to more than double that of all other T8 fluorescent lamp types, for example 3.5 mg in 2000 h in the case of a fluorescent lamp with a power consumption of 58 W. In addition, this leads to considerable graying and therefore poor maintenance.